ORIENTATION OF SECONDARY TERRESTRIAL ROOTS 
411 
soil cultures made their appearance. In view of the change of geotonus 
of secondary roots which Sachs,^ StahP and Czapek^ have reported as 
accompanying temperature changes, all the cultures in a single experi- 
ment were kept at the same temperature. During the course of all 
the experiments the temperature varied from 15° to 19° C. The 
cultures were kept in darkness except for short periods during which 
observations, drawings or photographs were being made, because, 
as Czapek^^ has shown, the geotonus of the secondary root changes 
when the root is illuminated. All cultures employed in a given 
experiment were exposed to light for the same length of time when 
the behavior of the roots was recorded. 
When secondary roots growing in air were so displaced that they 
formed a greater angle with the normal direction of the main root 
than their limiting angle, a curvature followed which, as Sachs has 
stated, involved the whole growing region. I also found, in accord- 
ance with Sachs's observations, that this curvature was subsequently 
considerably flattened and thereafter further permanent curvature 
was very slight or entirely lacking unless the root was still far removed 
from its limiting angle. The roots, however, maintained after the 
flattening of the first curvature a curvature or strong asymmetry of 
the terminal i to ij^ mm. The retention of this curvature or asym- 
metry of the tip was clearly dependent upon the maintenance of the 
turgor of the cells of the tip, for it could be caused to disappear and 
reappear repeatedly within a few minutes by alternately exposing the 
root to relatively dry air and spraying it with water. This curvature 
of the tip which is maintained by the root, although there may be 
scarcely any or indeed no permanent curvature, is shown in figure 2. 
It is similar to the curvature of the tip of the primary root which 
Nemec^^ first reported. As in the case of the primary root, this 
curvature of the extreme tip of the secondary root does not pass over 
as a permanent curvature to the region behind the tip, but is con- 
tinually being flattened and at the same time is being renewed by the 
cells of the growing point and the root cap. 
^ Sachs, 1. c. p. 624. 
s Stahl, Einfluss des Lichtes auf den Geotropismus einiger Pflanzenorgane. 
Bericht. Deutsch. Bot. Ges. 2: 396. 1880. 
9 Czapek, 1. c. p. 1251. 
1° Czapek, 1. c. p. 1250. 
11 Nemec, Ueber die Wahrnehmung des Schwerkraftreizes bei den Pflanzen. 
Jahrb. Wiss. Bot. 36: 93ff. 1901. 
